Undefined reference to 'function_name' in ARM assembly

Question

I'm attempting to put together some relatively simple code that uses two functions, fact and exp. However, whenever I try to branch to either of them, I get an error that reads: undefined reference to 'fact' and undefined reference to 'exp'. I tried adding .global fact and .global exp to the top of the file, but that didn't help. Here's my code:

.text
.global main
.extern printf

main:
    ldr x2,=x // load 'x' -> x value
    ldr d2,[x2]
    ldr x2,=n  // load 'n' -> stopping point
    ldr x3,[x2]
    mov x2,#1 // set i/exp value to 1
    scvtf d1,x2  // default d1 (return value) to 1.0
loop:
    subs xzr, x3, x2
    ble done
    sub sp, sp, #16
    str x2,[sp] // save the value of x2 first
    sub x7, x2, #1

    bl fact

    ldr x2,[sp]   // get original x2 value back
    add sp, sp, #16
    mov x5, #0
    fmov d4, d2

.func fact
    mul x2, x2, x7
    sub x7, x7, #1
    cbnz x7,fact
    mov x4, x2
    br x30
.endfunc

.func exp
    fmul d4, d4, d4
    add x5, x5, #1
    cmp x5, x2

    bl exp

    br x30
.endfunc

done:
    // done

.data
    n:
    .quad 5
    x:
    .double 1.45
.end

What assembler are you using? I suspect that `.func` doesn't do what you think it does. — Nate Eldredge, Dec 10 '20 at 20:30
In particular, if it's the GNU assembler, `.func` and `.endfunc` don't generate any code or labels, only debug info. You should just put a label `fact:` at the start of your function, and skip the `.func/.endfunc` unless you really know what you're doing as far as debug info. — Nate Eldredge, Dec 10 '20 at 20:32
By the way, I believe that in ARM64, one should normally use `ret` instead of `br x30` to return from a subroutine. Unlike on ARM32, they're actually different encodings, and I think `ret` is better for the branch predictor. — Nate Eldredge, Dec 10 '20 at 20:35
Oh, and your `bl exp` within `exp` will create an infinite loop. I think perhaps you meant `b.lt`. — Nate Eldredge, Dec 10 '20 at 20:38
@NateEldredge Yep, I meant blt. The code as a whole is still very much under construction. — TheFiveHundredYears, Dec 10 '20 at 20:40

score 2 · Accepted Answer · answered Dec 10 '20 at 20:47

In the GNU assembler, .func and .endfunc only serve to emit debugging info. They don't take the place of actually defining a label. (Nor do they emit any prologue or epilogue code, which might have been your next guess.)

So you should simply write, as for your other entry points:

fact:
    mul x2, x2, x7
    // ...

You might as well omit .func/.endfunc unless you are going to also produce proper debugging info for the rest of your code, which is usually not worth the trouble for handwritten assembly.

Undefined reference to 'function_name' in ARM assembly

1 Answers1